def Out_Excel(file_name,C,col):
    writer = pd.ExcelWriter(file_name,engine='xlsxwriter')
    for tab in tabs:    # tabs here is provided from a different function that I did not write here to keep it simple and clean
        df = DataFrame(C)    # the data is different for different sheets but I keep it simple in this case
        df.to_excel(writer,sheet_name = tab, startcol = 0 + col, startrow = 0)
    writer.save()

Out_Excel('test.xlsx',C,0)
Out_Excel('test.xlsx',D,10)

import pandas as pd
from openpyxl import load_workbook

book = load_workbook('test.xlsx')
writer = pd.ExcelWriter('test.xlsx')
writer.book = book
writer.sheets = dict((ws.title, ws) for ws in book.worksheets)

df.to_excel(writer, sheet_name='tab_name', other_params)

writer.save()